1. Field of the Invention
The present invention relates to a cathode-ray tube apparatus wherein leakage magnetic fluxes extending from a deflection yoke can be reduced.
2. Description of the Related Art
Unnecessary radiation, such as electronic waves, is controlled in accordance with the regulations such as VDE (Verband Deutscher Elektrotechniker). Generally, the leakage magnetic field of cathode-ray tubes are also controlled in accordance with VDE.
The recent trend is to limit leakage magnetic fields harmful to human being, particularly in Northern European countries, in accordance with MPR (SSI) regulations. Subjected to these regulations are magnetic fields of frequencies ranging from 1 KHz to 400 KHz. In the case of cathode-ray tubes, it is required to reduce, to a considerably low level, the intensity of leakage magnetic fluxes which are some of the magnetic fluxes generated by the horizontal deflection coil of the deflection yoke; and which do not serve to deflect the electron beams emitted from the electron gun assembly.
To control the leakage magnetic fluxes emanating from cathode-ray tube apparatuses, it is necessary to attenuate these fluxes in a predetermined manner. However, the leakage magnetic fluxes must not be attenuated in a manner that the effective magnetic fluxes are influenced to degrade the deflection characteristics of the deflection yoke, such as beam convergence and beam landing.
FIG. 1 is a perspective view showing a deflection yoke of popular type for use in a cathode-ray tube, such as a color cathode-ray tube. The deflection yoke comprises a molded member 1 and a pair of saddle-type main horizontal deflection coils 2 positioned in the member 1, symmetrically to each other with respect to the horizontal axis (i.e., X axis). Most of the magnetic fluxes generated by the coils 2, generally known as "effective magnetic fluxes," are confined in the deflection yoke, or within a hollow cylindrical core 3 which surrounds the molded member 1, and effectively serve to deflect electron beams in horizontal direction. The remaining magnetic fluxes, generally known as "leakage magnetic fluxes," radiate from the deflection yoke.
FIG. 2 is a diagram illustrating the distribution of the effective magnetic fluxes 6 and that of the leakage magnetic fluxes 7 and 10. As FIG. 2 shows, two reference leakage magnetic fluxes 8a and 8b emanate from the horizontal deflection coils 2 along lines defining an angle of 30.degree. to 40.degree.. As is evident from FIG. 2, the leakage magnetic fluxes 7, which extend from a flange portions 9 of the coils 2 substantially in parallel to the effective magnetic fluxes 6, exist in the region between the reference leakage magnetic fluxes 8a and 8b, whereas the leakage magnetic fluxes 10, which extends in the direction opposite to that of the effective magnetic fluxes 6, exit outside the region.
Various methods of controlling the leakage magnetic fluxes emanating from the outer periphery of the cathode-ray tube have been devised, one of which is to enclose the entire deflection yoke within a metal case. This method does not suffices to reduce the leakage magnetic fluxes to a desirable level. Further it is disadvantageous in two respects because of the use of the metal case covering the whole deflection yoke. First, a sufficient heat radiation is impossible. Secondly, the metal case is rather an expensive member and inevitably increases the manufacturing cost of the cathode-ray tube apparatus.
Published Unexamined Japanese Patent Application No. 62-64024 discloses a cathode-ray tube apparatus, in which as is shown in FIG. 3, a pair of auxiliary coils 11 having substantially the same shape as saddle-type main horizontal deflection coils 2 are located symmetrically to each other with respect to a core 3, opposing the main horizontal deflection coils, respectively. Part of the current flowing in either main horizontal deflection coil 2 is supplied to the corresponding auxiliary coil 11 in opposite phase, such that the auxiliary coils 11 generate magnetic fluxes (hereinafter referred to as "auxiliary magnetic fluxes") which extend opposite to the main magnetic fluxes emanating from the main coils 2 and which reduce the leakage magnetic fluxes also emanating from the main coils 2.
It is difficult, however, to control the auxiliary coils 11 accurately enough to reduce the leakage magnetic fluxes which exist in particular positions. Further, since the auxiliary coils 11 are located at the places where less leakage magnetic fluxes exist than other places, the leakage magnetic fluxes are reduced excessively at the rear of the deflection yoke, inevitably generating reverse leakage magnetic fluxes. The reverse leakage magnetic fluxes, thus generated in the vicinity of the yoke, are liable to influence the deflection characteristics of the deflection yoke.
As has been pointed out, the use of a metal case to reduce the leakage magnetic fluxes results in an inadequate heat radiation from the deflection yoke and also in an increase in the manufacturing cost of the cathode-ray tube apparatus, and the provision of auxiliary coils adversely influences the deflection characteristics of the cathode-ray tube apparatus.